The present invention relates to a method for obtaining a magnetization distribution image of a living body by measuring nuclear magnetic resonance signals such as those of hydrogen or phosphorus in a living body.
In an imaging apparatus which uses a nuclear magnetic resonance (NMR) phenomenon, it is necessary to separate and identify NMR signals from an article under test for each portion of the article. As one method therefor, a gradient magnetic field is applied to the article under test to generate different magnetic field intensities in which the portions of the article are placed which causes differences among resonance frequencies or amounts of phase encoding of the portions of the article so that positional information is obtained.
The latter is called a Fourier imaging method which is presently widely used. In this method, a Fourier transform is carried out in two stages (two-dimensional Fourier transform) or three stages (three-dimensional Fourier transform). There are two methods for displaying a magnetization distribution of the article under test. In one method, only real parts or imaginary parts of the result of reconstruction of an image obtained as complex numbers are displayed, and in the other method, absolute values of the result are displayed. In the former method, if there is a shift in start time of signal sampling, dark and light areas appear in an image. Accordingly, it is necessary to adjust timing of application of the gradient magnetic field or start timing of sampling prior to the measurement. In the latter method, the dark and light areas do not appear because of the absolute values and the level of background noise of the image can be set to the lowest level.
The magnetization distribution image usually represents a density distribution of a nuclear seed (e.g. hydrogen) under consideration. By improving a measurement sequence, the magnetization distribution image can represent a spin distribution image weighted by longitudinal relaxation time T.sub.1 of the nuclear seed (T.sub.1 enhanced image). From data of two magnetization distribution images having different longitudinal relaxation effects, T.sub.1 values at respective coordinates can be calculated and T.sub.1 distribution image can be obtained, as taught by PHYSICS IN MEDICINE AND BIOLOGY, Vol. 27, No. 8, pp 1057-1065 (1982). However, when those two magnetization distribution images are displayed by absolute values, an exact T.sub.1 value cannot be calculated unless the polarity of magnetization is anticipated because the positive magnetization and the negative magnetization are not distinguishable.